The present invention relates to optical scanning systems and, more specifically, to scanning systems and methods in which a light beam is scanned over a flat surface.
In optical scanning systems, such as those for use in printing, facsimile and data recording, an information containing, modulated light beam is directed onto a surface at which an image of the information is formed. It is desirable to be able to properly focus a light beam over the entire image surface to provide the desired degree of resolution for good image reproduction. The image surface, which may be photographic film or paper, in these systems may be either curved or flat. An example of a known optical scanning system utilizing a curved image surface is disclosed in the U.S. Pat. No. 3,520,586 to Bousky. It is desirable however to have an optical scanning system which can properly focus a light beam on a flat surface because, for example, a flat image surface is less complicated to implement than a curved image surface. More specifically, an image field which is not flat projected onto a plane will result in loss of resolution at certain points along the field. An attempt to correct this loss of resolution may be to curve the platen but this approach leads to an excessively complicated platen design and paper handling assembly.
Optical scanning systems are known wherein a light beam is scanned over a flat image surface. In a typically known optical scanning system, a beam of collimated radiation from a source is directed toward a scanning device, which in turn redirects the collimated light beam to a multi-element lens assembly that focuses the collimated light beam on a flat field. The multi-element lens assembly was considered necessary to provide the degrees of freedom needed to achieve the desired focal power and to correct for lens aberrations, such as astigmatism, spherical aberration, comma and petzval curvature. For example, the collimated beam of light, when scanned, appears to the focusing lens assembly as radiating from a flat surface. Therefore, the focusing lens assembly is required to overcome the petzval curvature effect in which a flat surface is imaged as a curved surface. As a result, the number of corrections required to be made in such an optical scanning system and the number of degrees of freedom available for optical design have necessitated a multi-element lens assembly if an undesirable compromise in the optical design is to be avoided.
However, the multi-element lens approach is expensive, not only due to the number of elements required, but also due to the added accuracy to which the lens surfaces are required to be polished. As a general rule, the larger the number of elements, the higher the degree of accuracy needed in surface polishing because the defects in such surfaces generally tend to be accumulative rather than canceling.
In the case of coherent light, such as that from lasers, a multi-element lens tends to create further problems because any imperfections in the surfaces create an undesirable scattering effect which results in undesirable beam interference patterns. In addition, the greater the number of optical elements, the more surfaces there are available for dust collection, which also scatters coherent light and also creates interference problems.